One example of a global navigation satellite system is the Global Positioning System GPS. The global position system (GPS) uses satellites orbiting the earth in known orbit paths with accurately known positions. These satellites transmit signals which can be received by a receiver on earth. Using signals received from four or more satellites, the receiver is able to determine its position using trigonometry. The signals transmitted by the satellite comprise pseudo-random codes. The accuracy of the determination of position is dependent on factors such as the repetition rate of the code, the components of the receiver and atmospheric factors.
The accuracy of measurement can be improved if the signals from more than four satellites are taken into account. GPS receivers can receive signals from up to twelve satellites at a time.
GALILEO is a European initiative for a global navigation satellite system which provides a global positioning service. It has been proposed that GALILEO be interoperable with GPS and GLONASS, the two other global satellite navigation systems. It should be appreciated that the term GNSS is used in this document to refer to any of these global positioning systems.
GALILEO has a system of thirty satellites, twenty-seven operational satellites with three operational in-orbit spares. The proposed frequency spectrum for GALILEO has two L-bands. The lower L-band, referred to as E5a and E5b, operate in the region of 1164 MHz to 1214 MHz. There is also an upper L-band operating from 1559 MHz to 1591 MHz.
In GPS, the frequency signals themselves are called the carriers. Since the carriers are pure sinusoids, they have two binary codes modulated onto them. The two binary codes are the C/A (coarse acquisition) codes and the P (precise) code.
Information on the coordinates of the satellite is also sent within the messages broadcast by the satellite and is modulated onto the carriers.
The coarse/acquisition (C/A code) was originally used as a coarse position measurement signal or as an acquisition code in order to assist locking onto the phase of the precise code. However, the C/A code is now used for acquisition and for position tracking. The C/A code is a pseudo-random (PN) binary code which consists of 1023 elements or chips that repeat every millisecond.
The C/A code is a Gold code. The cross correlation property of a Gold code is such that the correlation function between two different sequences is low. In other words, when a received signal is correlated with the correct C/A code the desired signal can be easily distinguished from other signals which appear as background noise.
In GPS and GALILEO, signals are broadcast from satellites which include these pseudo random codes which are processed at a receiver to determine position data. The processing involves first determining the relative offset of the received codes with locally generated versions of the codes (acquisition) and then determining the position once the relative offset is determined (tracking). Both acquisition and tracking involve correlating received signals with a locally generated version of the pseudo random codes over an integration period.
In spread spectrum systems, acquisition is difficult because it is two dimensional (frequency and time). A further difficulty is that because the signals are much weaker inside as compared to outside, it is much more difficult to acquire signals indoors. In particular, the indoor operation of GNSS requires the reception of signals attenuated by at least 20 dB from the outdoor equivalents.
The number of cells in the time domain is for example 2046. The number of cells in the frequency domain is 20 for outdoors at 1 KHz bandwidth or 2000 for indoors at 10 Hz bandwidth. This latter number of cells can be reduced to 20 with a temperature controlled crystal oscillator TCXO. This means that the total number of cells to be searched is 2046 times 20 i.e. around 40,000. For outdoors, each cell takes one millisecond and for indoors, each cell would take 100 milliseconds because of the weaker signal strength. This results in a search time of 40 seconds for outdoors or 4000 seconds for indoors, on a single correlator.
This problem traditionally is addressed by using parallelism in the frequency domain, for example sixteen fast Fourier transform channels or by parallelism in the time domain, using parallel correlators. To achieve parallelism requires faster clocks or more hardware which is disadvantageous. Additionally, more hardware or faster clocks require increased power.
In any event, the fundamental limit is the stability of the reference clock which prevents bandwidth reduction to the degree required for indoor sensitivity.
As already mentioned the indoor signals can be attenuated by at least 20 dB from their outdoor equivalents. To increase the sensitivity by twenty dB for the indoor signals means integrating for a hundred times longer. However, this is difficult to achieve because as the coherent integration period is extended, the bandwidth of the channel is narrowed. This in turn requires many more searches to be carried out and eventually the stability of the reference oscillator becomes a limiting factor as a signal appears to wander from one frequency to another, even before acquisition is completed. This results in a spreading of the energy, preventing further gain.
In addition, the modulation method used may provide a limit on the integration time.
Thus there are problems in performing integration with such signals. The integration time is limited by the accuracy of a local clock and the frequency shifts caused by relative motion of the satellite and receiver.
It is an aim of embodiments of the invention to address one or more of these problems.